As a single, useless data point, I'm not at all involved in the Mozilla community (though I've been a user for a very long time), or in the same-sex marriage fight, nor am I a resident of California. And my first thought when I heard the announcement was basically "really? The Prop 8 guy?"
This is pretty basic stuff for high-profile executive appointments...
I agree that it should have come up in the discussion; I think that even if it did, it is a reasonable decision to think that his appointment as a whole would be worth the risk.
That's really what this was -- a calculated risk, just like all CEO appointments. I think it's just as reasonable to believe that it could have gone the other way -- that there wouldn't have been as big of a backlash.
Hubris? Nah. They just misjudged the ability of the media and vocal critics to not spin a historical donation as a wildly out of scope controversy for the sake of attention, pageviews, and a misguided sense of social rights.